1. Field of the Invention
The present invention relates to a developing apparatus, a developing method and magnetic toner which are used for an image-forming method for visualizing electrostatic images in electrophotography.
2. Description of the Related Art
Image-forming apparatuses such as copiers and printers recently have varied usage purposes and environments as well as being required to be even faster, provide higher image quality and have higher stability. For example, printers which have conventionally been mainly used in offices are now used in harsh environments and thus it is important for the printers to provide stabilized images under such conditions. Known developing apparatuses used for such image-forming apparatuses generally comprise the configuration in which a blade made of rubber or metal acting as a toner layer thickness regulating member for regulating the toner coat amount is brought into contact with the surface of a developing sleeve acting as a developer (toner)-carrying member.
Toner is provided with positive or negative charge by friction between the regulating member and toner and/or friction between the toner-carrying member and toner. The charged toner is thinly applied on the surface of the toner-carrying member by the regulating member. It is a general developing method in which the charged toner is allowed to fly and adhere to an electrostatic latent image on the surface of an electrostatic latent image bearing member opposing to the toner-carrying member.
It is recently required that the image-forming apparatus technology is directed to provide high fineness, high quality and high image quality as well as high speed and high reliability for a long term use. On the other hand, in terms of energy saving, it is also required to provide better fixing performance at lower temperatures. Under such circumstances, depending on the type of materials of regulating members, usage environments or image printing conditions such as process speed, toner may be melt-adhered on various members in developing apparatuses, resulting in image defects such as image density non-uniformity or streaks.
When, in a high-temperature, high-humidity environment which is particularly disadvantageous for rising of charging toner, continuous image printing is carried out for a long term, some magnetic toner in the developing apparatus which has faster rising of charging may be sometimes consumed preferentially, namely, so-called selective development may occur. As a result, when the developing apparatus is used again for image printing after it has been left to stand during the latter half of the usage, images may be deteriorated by, for example, low density and fogging.
On the other hand, various trials for improving regulating members and toner-carrying members have been carried out. Japanese Patent Application Laid-open No. 2004-4751 discloses the hardness and deformation rate on the surface of a developer bearing member and a developing apparatus in which the ten point mean roughness (Rz) of the surface which contacts the developer bearing member of a developer amount regulating blade is 0.3 to 20 μm. In this patent document, non-magnetic black toner is evaluated on the developing apparatus and exhibits effects on solid image density, unevenness and streaks in each environment. On the other hand, stability in a long term durability test has not been sufficiently evaluated, and in particular, when monocomponent magnetic toner is used, the effects tend to be insufficient.
Japanese Patent Application Laid-open No. 2007-79118 discloses a trial for improving toner melt adhesion and thin line reproducibility by using a specific toner regulating blade to define the adhesion strength between the toner regulating blade and toner. However, in this document, material of the blade or the amount of an external additive(s) is not sufficiently optimized, so that there is room for improvement in terms of low density or fogging which particularly occurs after a long term durability test.
Therefore, there has been a need for a developing apparatus comprising monocomponent magnetic toner which is stable in a long term durability test and can provide preferable images with superior image density and less fogging even when it is left to stand after a long term durability test.